Among fluorides, a calcium fluoride (CaF2) single crystal body, a magnesium fluoride (MgF2) single crystal body and the like have been used in the optical field, for example, in the vacuum ultraviolet region of wavelengths of 160 nm or less, or in the extreme infrared region of wavelengths of 3 μm or more. They have been used as a lens, a prism and the like for such specific wavelength regions wherein light does not pass through the glass members widely used in the market such as a very-high-purity quart glass and optical glasses. Therefore, they are naturally expensive optical members.
Generally speaking, there are very few cases where a fluoride is used for other than such optical uses. The CaF2 single crystal body, a lithium fluoride (LiF) single crystal body, or an aluminum fluoride (AlF3) single crystal body has been rarely used as a shield to neutrons, one of radioactive rays. However, such single crystal bodies have plane orientation dependency of moderation performance originated in crystal orientation and ununiformity due to structural defects such as subgrains, and moreover, they are extremely expensive.
The radioactive rays are roughly classified into alpha (α)-rays, beta (β)-rays, gamma (γ)-rays, X-rays, and neutrons. The power passing through a substance (penetrability) gradually increases in this order.
The neutrons which have the highest penetrability among them are further classified into the below-described groups, for example, according to the energy level which they have. The energy each type of neutrons has is shown in parentheses, and the larger the value is, the higher the penetrability is.
In the order of the lowest penetrability, they are classified into cold neutrons (up to 0.002 eV), thermal neutrons (up to 0.025 eV), epithermal neutrons (up to 1 eV), slow neutrons (0.03-100 eV), intermediate neutrons (0.1-500 keV) and fast neutrons (500 keV or more).
Here, there are various views concerning the classification of neutrons, and the energy values in the parentheses are not precise. For example, there is a view that mentions 40 KeV or less, which is within the above energy region of intermediate neutrons, as the energy of epithermal neutrons.
The typical effective utilization of neutrons is an application to the medical care field. In particular, the radiation therapy in which tumor cells such as malignant cancers are irradiated with neutrons so as to be broken has been coming into general use in recent years.
In order to obtain medical effects in the present radiation therapy, neutrons of a certain high energy must be used, so that in the irradiation of neutrons, the influence on a healthy part other than an affected part of a patient cannot be avoided, leading to side effects. Therefore, in the present situation, the application of the radiation therapy is limited to severe patients.
When a normal cell is exposed to high-energy neutrons, its DNA is damaged, leading to side effects such as dermatitis, anemia due to radiation and leukopenia. Furthermore, in some cases, a late injury may be caused some time after treatment, and a tumor may be formed and bleed in the rectum or the urinary bladder.
In recent years, in order not to cause such side effects and late injuries, methods of pinpoint irradiation on a tumor have been studied. Examples thereof are: “Intensity Modulated Radiation Therapy (IMRT)” in which a tumor portion is three-dimensionally irradiated accurately with a high radiation dose; “Motion Tracking Radiation Therapy” in which radiation is emitted to motions in the body of a patient such as breathing or heartbeat; and “Particle Beam Radiation Therapy” in which a baryon beam or a proton beam each having a high remedial value is intensively emitted.
The half-life of a neutron is short, about 15 min. The neutron decays in a short period of time, releases electrons and neutrinos, and turns into protons. And the neutron has no charge, and therefore, it is easily absorbed when it collides with a nucleus. The absorption of neutrons in such a manner is called neutron capture, and one example of an application of neutrons to the medical care field by use of this feature is the below-described “Boron Neutron Capture Therapy (hereinafter, referred to as BNCT)”, a new cancer therapy which is recently gaining attention.
In this BNCT, by causing tumor cells such as malignant cancers to react with a boron drug which is injected into the body by an injection, a reaction product of a boron compound is formed in the tumor portion.
The reaction product is then irradiated with neutrons of an energy level which has less influences on a healthy part of the body (desirably comprising mainly epithermal neutrons, and low-energy-level neutrons being lower than epithermal neutrons). And a nuclear reaction with the boron compound is caused only within a very small range, resulting in making only the tumor cells extinct.
Originally, cancer cells easily take boron into them in the process of vigorously increasing, and in the BNCT, by use of this feature, only the tumor portion is effectively broken.
This method was proposed about 60 years ago. Because of small influences on a healthy part of a patient, it has been attracting attention as an excellent radiation therapy since quite long before and has been researched and developed in varied countries.
However, there are wide-ranging important problems on the development such as development of a neutron generator and a device for a selection of the types of neutrons to be remedially effective, and avoidance of influences on a healthy part other than an affected part of a patient (that is, formation of a boron compound only in a tumor portion). Therefore, the method has not come into wide use as a general therapy. Significant factors in terms of apparatus why it has not come into wide use are insufficient downsizing of the apparatus and insufficient enhancement of its performance.
For example, there is a latest system of the BNCT, which a group with Kyoto University as the central figure has been promoting (Non-Patent Document 1 and Non-Patent Document 2). This system comprises an apparatus for medical use only, having a cyclotron accelerator as a neutron generator which is exclusively installed without being attached to an existing nuclear reactor.
One report says that the accelerator alone weighs about 60 tons, and its size is quite large. In the cyclotron system, protons are accelerated by use of a centrifugal force in a circular portion of the cyclotron and caused to collide with a target metal such as a plate made of beryllium (Be) so as to generate fast neutrons. In order to efficiently generate neutrons, it is required to make the diameter of the circular portion large so as to obtain a large centrifugal force. That is one of the reasons why the apparatus is large.
Furthermore, in order to safely and effectively utilize the generated radiation (mainly neutrons), a radiation shield such as a shielding plate (hereinafter, referred to as a moderator) is required. As moderators, polyethylene containing CaF2 or LiF, as well as Pb, Fe, Al and polyethylene, are selected. It cannot be said that the moderation performance of these moderators is sufficient, and in order to conduct required moderation, the moderator becomes quite thick. Therefore, the moderation system device portion including the moderator is also one of the reasons why the apparatus is large.
In order to allow this BNCT to come into wide use in general hospitals, hereinafter, downsizing of the apparatus is necessary. In addition to further downsizing of the accelerator, to improve the remedial values by developing a moderator having high moderation performance and achieve downsizing of the moderation system device by the improvement of moderation performance is an urgent necessity. The moderator which is important for downsizing a BNCT apparatus and improving remedial values is described below.
As described above, in order to safely and effectively utilize radiation, it is necessary to arrange a moderator having the right performance in the right place. In order to effectively utilize neutrons having the highest penetrability among radioactive rays, it is important to accurately know the moderation performance of every kind of substances to neutrons so as to conduct effective moderation.
One example of the selection of particle beam types in order to effectively utilize neutrons for medical care is shown below.
By removing high-energy neutrons which adversely influence the body (such as fast neutrons and a high-energy part of intermediate neutrons) as much as possible, and
by further reducing extremely-low-energy neutrons having little medical effect (such as thermal neutrons and cold neutrons), the ratio of neutrons having high medical effects (such as a low-energy part of intermediate neutrons and epithermal neutrons) is increased.
As a result, a particle beam effectively utilized for medical treatment can be obtained. The low-energy part of intermediate neutrons and epithermal neutrons have a relatively high invasive depth to the internal tissues of a patient. Therefore, for example, in the case of irradiating the head with the low-energy part of intermediate neutrons and epithermal neutrons, without craniotomy required, as far as the tumor is not present in a considerably deep part, it is possible to carry out effective irradiation to an affected part in an unopened state of the head.
On the other hand, when the extremely-low-energy neutrons such as thermal neutrons are used in an operation, because of their low invasive depth, craniotomy is required, resulting in a significant burden on the patient.
In order to improve remedial values in the BNCT, it is required to irradiate an affected part with a large quantity of neutrons comprising mainly epithermal neutrons and some thermal neutrons.
Specifically, an estimated dose of epithermal neutrons and thermal neutrons required in cases where the irradiation time is in the order of one hour, is about 1×109 [n/cm2/sec]. In order to secure the dose, it is said that as the energy of an outgoing beam from an accelerator being a source of neutrons, about 5 MeV-10 MeV is required when beryllium (Be) is used as a target for the formation of neutrons.
The selection of particle beam types through moderators of every kind in a neutron radiation field for BNCT using an accelerator is described below.
A beam emitted from the accelerator collides with a target (Be, in this case), and by nuclear reaction, high-energy neutrons (fast neutrons) are mainly generated. As a method for moderating the fast neutrons, using lead (Pb) and iron (Fe) each having a large inelastic scattering cross section, the neutrons are moderated to some extent. In order to further moderate the neutrons moderated to some extent (approximately, up to 1 MeV), optimization of the moderator according to the neutron energy required in the radiation field is required.
As a moderator, aluminum oxide (Al2O3), aluminum fluoride (AlF3), calcium fluoride (CaF2), graphite, heavy water (D2O) or the like is generally used. By injecting the neutrons moderated nearly to 1 MeV into these moderators, they are moderated to the epithermal neutron region of the energy suitable for BNCT (4 keV-40 keV).
In the case of the above Non-Patent Document 1 and Non-Patent Document 2, as moderators, Pb, Fe, polyethylene, Al, CaF2 and polyethylene containing LiF are used. The polyethylene and polyethylene containing LiF among them are used as moderators for safety (mainly for shielding) which cover the outside portion of the apparatus in order to prevent leakage of high-energy neutrons out of the radiation field.
It can be said that it is appropriate to moderate the high-energy part of neutrons to some extent using Pb and Fe among these moderators (the first half of the stage of moderation), but it cannot be said that the second half of the stage of moderation using Al and CaF2 after the moderation to some extent is appropriate.
That is because the moderators used in the second half of the stage thereof has insufficient shielding performance to fast neutrons, and a high ratio of fast neutrons having a possibility of bad influences on healthy tissues of a patient is left in the moderated neutron type.
By reason of CaF2 having insufficient shielding performance to the high-energy part of neutrons as a moderator used in the second half of the stage thereof, part of them passes without being shielded.
The polyethylene containing LiF used with CaF2 in the second half of the stage thereof covers over the entire surface except an outlet of neutrons on the treatment room side. It is arranged so as to prevent whole-body exposure of a patient to the fast neutrons, without having a function as a moderator on the outlet of neutrons.
For information, the polyethylene among the moderators in the first half of the stage thereof covers over the entire surface of the periphery of the apparatus except the treatment room side, like the polyethylene containing LiF in the second half of the stage thereof, and it is arranged so as to prevent the fast neutrons from leaking to the surroundings of the apparatus.
Therefore, instead of CaF2 as a shielding member to fast neutrons in the second half of the stage thereof, the development of a moderator which can shield and moderate high-energy neutrons while suppressing the attenuation of intermediate-level-energy neutrons required for treatment has been desired.
From various kinds of researches/studies, the present inventors found a MgF2 sintered body or MgF2 system substances, more specifically, a MgF2—CaF2 binary system sintered body as a moderator which makes it possible to obtain neutrons (neutrons of the energy of 4 keV-40 keV) mainly comprising epithermal neutrons in anticipation of the highest remedial value, from the above neutrons moderated to some extent (the energy thereof is approximately up to 1 MeV). As the MgF2 system substances, a MgF2—LiF binary system sintered body, a MgF2—CaF2—LiF ternary system sintered body other than the MgF2—CaF2 binary system sintered body can be exemplified.
As of now, there has been no report that magnesium fluoride (MgF2) was used as a moderator to neutrons, not to mention that there has been no report that a MgF2 sintered body or a MgF2—CaF2 binary system sintered body was used as such neutron moderator.
The present inventors has filed an application of an invention relating to a sintered body of MgF2 simple (a technical term related to raw material technology, a synonym for “single”) prior to this invention (Patent Document 1: Japanese Patent Application No. 2013-142704, hereinafter, referred to as the prior application).
MgF2 is a colorless crystal, having a melting point of 1248° C., a boiling point of 2260° C., a density (i.e. true density) of 3.15 g/cm3, a cubic system and a rutile structure according to a science and chemistry dictionary. On the other hand, CaF2 is a colorless crystal, having a melting point of 1418° C., a boiling point of 2500° C., a density (i.e. true density) of 3.18 g/cm3, a Moh's hardness of 4, a cubic system and a fluorite structure.
A single crystal body of MgF2 has high transparency, and since high light transmittance is obtained within a wide range of wavelengths of 0.2 μm-7 μm and it has a wide band gap and high laser resistance, it has been mainly used as a window material for eximer laser. Or when a MgF2 single crystal body is deposited on the surface of a lens, it shows effects of protection of the inner parts thereof or prevention of irregular reflection. In either case, it is used for optical use.
On the other hand, since the MgF2 sintered body has low transparency because of its polycrystalline structure, it is never used for optical use. Since the MgF2 sintered body has high resistance to fluorine gas and inert gas plasma, a few patent applications concerning an application thereof to a plasma-resistant member in the semiconductor producing process have been filed. However, there is no publication or report that it was actually used in the semiconductor producing process. That is because the MgF2 single crystal body has a strong image of extremely high price and the MgF2 sintered body produced by a general method has low mechanical strength as described in the below-mentioned Patent Document 2.
As for a MgF2 sintered body, according to the Japanese Patent Application Laid-Open Publication No. 2000-302553 (the below-mentioned Patent Document 2), the greatest defect of ceramic sintered bodies of fluoride such as MgF2, CaF2, YF3 and LiF is low mechanical strength. And in order to solve this problem, the invention was achieved, wherein a sintered body compounded by mixing these fluorides with alumina (Al2O3) at a predetermined ratio can keep excellent corrosion resistance of the fluorides as well as obtain high mechanical strength.
However, as for the corrosion resistance and mechanical strength of the sintered bodies produced by this method, in any combination, the sintered bodies are simply allowed to have just an intermediate characteristic between the characteristic of any of the fluorides and that of alumina. No sintered body having a characteristic exceeding one's characteristic superior to the other's has been obtained by compounding. In addition, their use is limited to high corrosion resistance uses, greatly different from the uses of the present invention.
Another sintered body mainly comprising MgF2 is described in Japanese Patent Application Laid-Open Publication No. 2000-86344 (the below-mentioned Patent Document 3), but its use is also limited to a plasma-resistant member. In the Patent Document 3, a sintered body comprises a fluoride of at least one kind of alkaline earth metals selected from the group of Mg, Ca, Sr and Ba, in which the total amount of metallic elements other than the alkaline earth metals is 100 ppm or less on a metal basis, the mean particle diameter of crystal grains of the fluoride is 30 μm or less, and the relative density is 95% or more.
However, the materials in the list (Table 1) of Examples of the Patent Document 3 were obtained by firing a fluoride of each single kind of the above four alkaline earth metals (i.e. MgF2, CaF2, SrF2 and BaF2), and no fired mixture of those fluorides is described.
Still another example of an application of a sintered body mainly comprising MgF2 to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2012-206913 (the below-mentioned Patent Document 4). The Patent Document 4 discloses an invention wherein, since a sintered body of MgF2 simple has a defect of low mechanical strength, by mixing at least one kind of non-alkaline metallic dispersed particles having a lower mean linear thermal expansion coefficient than MgF2 such as Al2O3, AlN, SiC or MgO, the defect of low mechanical strength thereof can be compensated for.
However, when a sintered body of such mixture is used as the above moderator to neutrons, the moderation performance thereof is greatly different from that of MgF2 simple because of the influence of the non-alkaline metal mixed into MgF2. Therefore, it is easily predicted that it is difficult to apply a sintered body of this kind of mixture to a use as a moderator.
In addition, an example of an application of a sintered body mainly comprising CaF2 to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2004-83362 (the below-mentioned Patent Document 5). The Patent Document 5 describes a method wherein using hydrofluoric acid, impurities other than Mg is removed from a low-purity raw material containing Mg, so as to precipitate high-purity CaF2, and a fluoride sintered body whose starting raw material is the high-purity CaF2 containing Mg of 50 ppm or more and 5% by weight or less is produced. A problem here is a state of Mg contained in the starting raw material, though the state is not described at all. And there is no description concerning the technique by which the degree of purity of the low-purity raw material is raised using hydrofluoric acid.
Then, when presuming the process of raising the degree of purity of a low-purity raw material as a person skilled in the art, generally speaking, in the case of raising the degree of purity of a low-purity raw material using hydrofluoric acid, a method is often adopted, wherein impurities in the raw material are first dissolved into a hydrofluoric acid solution as many as possible, and if a component (Ca, here) desired to be a main raw material dissolved with impurities in this dissolution process, the component is precipitated and separated by use of the difference in solubility among the dissolved components.
When further reviewing the invention, it is presumed that Mg exhibited different dissolution behavior from other impurities. In the specification, it is referred to as only “Mg”, and according to the descriptions of Examples in Table 1, as for high concentrations of impurity components other than Mg (such as Fe, Al, Na and Y), all of their concentrations were decreased by purity raising treatment, but only the concentration of Mg did not change, being 2000 ppm before the treatment and being also 2000 ppm after the treatment.
Hence, there is a high possibility that Mg might be in a state which is hard to dissolve in hydrofluoric acid, that is, a metal state. If CaF2 containing metal Mg is a starting raw material, the sintering process thereof is very different from the case like the present invention wherein a mixture of CaF2 and MgF2 is a starting raw material, and the characteristics of the sintered bodies are also very different from each other.
On the other hand, an invention relating to a neutron moderator was disclosed lately. That is the Japanese Patent No. 5112105 (Patent Document 6). The Patent Document 6 discloses ‘a moderator which moderates neutrons, comprising a first moderating layer obtained by melting a raw material containing calcium fluoride (CaF2), and a second moderating layer comprising metal aluminum (Al) or aluminum fluoride (AlF3), the first moderating layer and the second moderating layer being adjacent to each other’.
In the Patent Document 6, the first moderating layer obtained by melting the raw material containing CaF2 is disclosed, but raw material conditions such as the purity, components, particle size and treatment method thereof, and melting conditions such as heating temperatures, holding times thereof and the type of heating furnace are not mentioned at all, very insincerely described as a patent specification. In the Patent Document 6, there is no description suggesting that something related to MgF2 should be used as a neutron moderator.
In the preceding documents, as described above, there is no description suggesting the use of a sintered body of MgF2 as a moderator to neutrons, one kind of radiation. In such situation, the present inventors found that it was possible to use a MgF2 sintered body with a modification made thereto as a moderator to neutrons, one kind of radiation, and achieved the invention of the prior application.
In the invention of the prior application, a high-purity MgF2 raw material is pulverized and two-stage compressing and molding step is conducted thereon. That is, after molding by a uniaxial press molding method, this press molded body is further molded by a cold isostatic pressing (CIP) method so as to form a CIP molded body.
Then, by firing the same with three-level heating conditions using an atmosphere-adjustable normal pressure furnace, a sintered body having a compact structure is produced with suppressing foaming of MgF2 as much as possible.
However, since MgF2 very easily generates foams, it is not easy to actually suppress its foaming. As a result, the range of relative densities (i.e. 100×[bulk density of a sintered body]/[true density](%)) of sintered bodies produced by this method was 92%-96%, and the mean value thereof was of the order of 94%-95%.
A characteristic desired for a sintered body for a neutron moderator is ‘the mean value of relative densities of 95% or more at least, desirably 96% or more should be stably secured’.
In order to achieve the characteristic, the present inventors worked toward further development and found that when using a MgF2—CaF2 binary system sintered body for radiation use, the relative density thereof could be easily improved, compared with a sintered body of MgF2 simple, and that by making the sintering conditions proper, sintered bodies having more desirable densities could be stably produced, leading to the completion of the present invention.